1. Field of the Invention
The invention in general relates to crystal growth and more particularly to the preparation of single crystal silicon carbide boules having very low resistivity, from which semiconductor substrates may be fabricated.
2. Description of Related Art
Silicon carbide is being extensively explored as a semiconductor material for various electronic applications. Semiconductor devices of silicon carbide have the ability to operate at higher voltages and temperatures than conventional silicon or gallium arsenide devices thus providing for higher power devices with reduced cooling requirements. Electronic applications for silicon carbide semiconductor devices include compact, low cost airborne, ship and ground radars, aircraft engine and flight controls, electric tank and ship propulsion systems and satellite communications, to name a few. In the commercial sector, silicon carbide may be used in the field of high definition television, industrial power conditioning, nuclear control and instrumentation and electric vehicle power trains.
Silicon carbide crystals may be grown in a variety of ways, one of which utilizes a seed crystal of silicon carbide positioned within a furnace which also includes a source of pure silicon carbide. The furnace is heated to a temperature whereby the silicon carbide sublimates and is deposited upon the seed crystal. In one process, known as physical vapor transport, the seed crystal is positioned within a container having a growth cavity for the crystal. As silicon carbide is deposited from the source, a silicon carbide crystal, called a boule, is grown within the container. The physical vapor transport process is described in articles by D. L. Barrett et. al. in Journal of Crystal Growth Vol. 109 (1991) pp. 17-23, and Vol. 128 (1993) pp. 358-362, both of which are hereby incorporated by reference.
The grown boule is sliced into wafers for use as semiconductor device substrates. Many of these devices are vertically conducting devices, such as VMOS power switches and static induction transistors which require highly conductive substrates to minimize ohmic losses and to provide efficient performance. The method of the present invention results in an extremely low resistivity silicon carbide crystal which may be advantageously used as a substrate in silicon carbide semiconductor devices.